In the conventional formation of cigarettes, a filler stream having variations in quantities of tobacco in its length is conveyed on an air previous band under the influence of vacuum applied to the band to a trimmer where any tobacco above a predetermined distance from the conveyor surface is trimmed from the stream. The trimmed filler rod then is compressed to a uniform diameter prior to wrapping in paper to form a continuous cigarette rod from which discrete cigarettes are formed.
Unfortunately, in this procedure, the vacuum applied to the filler stream causes the density of tobacco to vary along the length of the stream, the variations depending on the quantity of tobacco in the stream. To compensate for these variations, in an attempt to provide a uniform density cigarette filler rod, the prior art has used one of two methods. In the first such method, the trimmer is moved relative to the conveying band to vary the distance of the trimmer from the conveying band in response to upstream-measured quantity and/or density variations. The second method involves maintaining the trimmer in a fixed position relative to the conveying band so that the distance between the conveying band and the trimmer is substantially constant while the vacuum applied to the conveying band and hence to the filler stream is varied in response to upstream-measured variations in quantity and/or density of the stream.
The aim of both these prior art methods is to vary the quantity of tobacco trimmed from the stream in response to variations in the density of the untrimmed stream, in order to provide a uniform quantity cigaratte filler rod for formation into cigarettes.
The prior art methods, while successful to a degree in improving the uniformity of the quantity of the cigarette filler rod, suffer from the drawback that relatively long lengths of the stream are subjected to adjustment or correction for density variations and the density adjustments can only be made as an average over a particular length of stream and hence density and resulting quantity variations remain. This effect arises from the lack of rapid response of trimming adjustment devices compared to abrupt variations in the high speed tobacco stream. Hence, the prior art methods are incapable of providing a completely uniform quantity cigarette filler rod.
Further both these prior art methods assume that the compression characteristics of the tobacco are uniform along the length of the stream.
However, this is often not the case, but such variations tend on average to cancel each other out. However, when the correction in the quantity of tobacco trimmed is made in accordance with the upstream-determined density and/or quantity variations, to provide the uniform quantity rod after trimming, the operation will result in hardness variations in the trimmed rod. Hence, even if a uniform quantity filler rod were provided by the prior art, variations in hardness of the filler rod often remain, leading to the provision of cigarettes of inconsistent quality.
In addition to these drawbacks, the prior art methods require separate detection and correction operations, necessitating the use of complicated and sophisticated apparatus.